RELEVANCE TO PUBLIC HEALTH: Congenital heart defects (CHD) are the leading cause of morbidity and mortality in infants. This proposal presents a unique approach to identifying the genetic basis of congenital heart defects (CHD) using a study population as the basis of a sensitization strategy to identify genetic and environmental risk factors for CHD. GOALS AND RELEVANCE TO NHLBI: Individuals with Down syndrome (DS) have a 2000-fold increased risk of AVSD. While increased dosage of chromosome 21 genes clearly contributes to this risk, AVSD occurs in only 20% and not 100% of DS individuals (and only 50% have any heart disease), demonstrating that trisomy 21 itself is not sufficient to cause CHD. Thus additional genetic variation and/or environmental factors influence this outcome. The high risk DS population can be used not only to study the contributions of dosage-sensitive chr21 genes in the etiology of CHD, but also serves as a unique "sensitized" population in which to identify risk factors for AVSD in the general population. We have already acquired DMA, cell lines, environmental questionnaire information and clinical data for more than 120 individuals with DS and heart disease (DS + AVSD) and their parents, plus a larger population of DS without heart disease (DS - CHD), the largest study set for this problem in existence. Using our existing infrastructure, we will expand this set to 600 DS + AVSD families to provide an adequate baseline study set for a genome-wide association study. We will assess candidate genes by targeted association studies and/or by resequencing in individuals with DS + AVSD and in appropriate controls. An interaction between one candidate gene and trisomy has already been identified in our previous work. Candidate mutations will be recreated in mouse and crossed to a trisomic background;these models provide access to all tissues at all developmental stages and can be manipulated genetically, providing tools that are essential to a full understanding of the etiology of CHD, and for initial studies of ameliorative interventions. Our goals are to establish study populations for a comprehensive analysis of AVSD and query candidate modifier genes in these populations. We will: 1. Conduct association studies on candidate genes to identify common variants that lead to CHD susceptibility;2. Identify rare susceptibility variants by resequencing AVSD-associated genes. 3. Use mouse models to test the roles in heart septation of candidate gene mutations in euploid and trisomic mice. This combination of approaches will allow us to identify susceptibility genes for all CHD and provide a system to establish precisely the etiology of defects observed through development. These systems will be invaluable in initial studies of ameliorative of interventions in CHD.